1. Field of the Invention
The present invention relates to a plastics molding system and an optical element formed by the plastics molding system. More particularly, the present invention relates to a plastics injection molding system using a plurality of molds, which is so-called a rotary molding system. According to the rotary molding system, the site where the molds are cooled is different from where the injection filling is performed.
2. Description of the Related Art
When a plastic product is formed by injection molding, normally, after a mold is set on an injection molding machine, a heated and molten resin is injected into the mold. When the resin is cured by cooling, the cured resin is taken out from the mold. Then, the next molten resin is injected into the empty mold. This processing is repeated.
However, according to a molding using such processing, it is necessary to take long cooling time when molding thick products or high precision products used, for example, in optical systems such as plastic lenses, prisms, and mirrors. While the resin is cured, which takes long time, the mold cannot be used. Thus, the injection molding machine cannot be used for the injection of the next resin while the injected resin is being cured. For this reason, the usage ratio of the injection molding machine is not good and productivity is not increased.
To alleviate the above-discussed limitations, a plastics injection molding system referred to as a rotary molding system has been proposed, for example, in Japanese Patent Application Laid-Open No. 05-124078 (JP 05-124078). According to the system described in JP 05-124078, a plurality of molds is prepared and the molds are sequentially loaded onto an injection molding machine, so that molten resin is successively injected into the molds one at a time. When molten resin is injected into a first mold, the first mold filled with the resin is moved to another place and cooled. Immediately after the first mold filled with the resin is moved to another place, molten resin is injected into the next mold loaded on the injection molding machine. When the first mold filled with resin has cooled, the molded form is extracted. Each process of heating and injecting the resin into a mold, and cooling and extracting the molded form is repeated at a specified processing time known as the “takt time”.
In the following description, a plastics molding system that performs the injection filling processing and the cooling processing at different sites from each other using a plurality of molds, such as the system described above, is referred to as a rotary molding system.
In recent years, with advance in image quality of digital copying machine and printers, precision at a higher level has been required for optical elements. Under such circumstances, demands for not only optical elements of high precision shapes but also elements of higher optical characteristics regarding, for example, birefringence and refractive index distribution has been increasing. The rotary molding is useful not only in obtaining molded products having high precision shape but in obtaining molded products having good optical characteristics as above described in a short takt time.
For example, in order to obtain molded products with good refractive index distribution, it is effective to slow down the cooling speed of the mold at a temperature near the glass transition temperature. Further, in order to reduce the birefringence, it is effective to set high mold temperature. Since longer cooling time is necessary if the mold cooling speed is decreased and higher mold temperature is used, the efficiency in the use of the molding machine is reduced.
According to the above-described rotary molding system, since the injection-filled mold is moved and cooled in another place, molded products with good optical characteristics can be obtained in a short takt time without reducing the efficiency in the use of the molding machine. However, there are problems in using the rotary molding system. Specifically, although molded products having high precision shape and high precision optical characteristics can be produced in a short takt time, a number of molds that require long cooling time and short takt will be needed. In recent years, with the demand for high precision molded products, high precision shape molds have been also demanded. Thus, it is necessary to prepare a number of high-precision molds in a short takt time.